When using the items endpoint to create a single Item, you pass 1 field only: the url. This allows Taboola to crawl the URL and extract the relevant metadata for the new Item.
The mass endpoint allows you to create a batch of campaign items - each item with a url, title and thumbnail that you provide. The endpoint saves these values without crawling the URL.
Make sure that the 3 values provided are acceptable to Taboola.